Highlights: STC is your north star. Every soundproofing decision should be guided by Sound Transmission Class ratings. A standard wall sits at STC 33 (nearly useless for privacy); you need STC 45 for basic privacy, STC 50+ for multi-family or media use, and STC 55+ for music or recording work. Four principles drive all soundproofing: mass, decoupling, absorption, and damping. No single material covers all four — the best results come from layering them together strategically. Seal before you spend. Flanking paths through outlets, door gaps, and trim can completely undermine an expensive wall build. Acoustical sealant costs a fraction…